Funk ACES Library, University of Illinois, 2006.
http://www.youtube.com/watch?v=cwihz7iZlx0

Addendum, April 1, 2014 (thanks, Steve Lawson!) The London Public Library in California put together a similar shenanigan in March of 2014:

Williams College Libraries, Williamstown, Massachusetts. Library-centered spoof of the A-Team. http://www.youtube.com/watch?v=YwCUtpbUWgk

A Star Wars Adventure. Whittier College Library, March 16, 2006. A light sabre battle in the stacks. http://www.youtube.com/watch?v=b2WxwzGY42Q

Betty Glover Library Workout Tape Ad.
Hayden Library, Arizona State University, 1987.
http://www.youtube.com/watch?v=_k8BKX2eQ0Q

Many libraries, many places, time immemorial-ongoing.
Students at many colleges and universities claim that sex in the stacks is common, sometimes even a rite of passage before graduation. It is unknown how much actual sex in libraries takes place, but a review of the literature and an informal poll suggest that sex in the stacks is more of a myth than a reality. http://www.thecrimson.com/article.aspx?ref=505583, http://library.duke.edu/blogs/answerperson/perkins-lobby-edition/

Streaking. library unknown, 1970s. Image from The Library in America (Facts on File, 1986). http://www.txla.org/pubs/tlj75_2/libs.html

UC-Berkeley library, finals week, 2008. http://sfist.com/2008/12/22/naked_ucberkeley_streakers_jiggle_f.php

Norlin Library, University of Colorado – Boulder, 2004 (an annual sorority tradition?) http://www.collegehumor.com/video:40838 WARNING: you’ll have to register to see this (rather tame) streak, and the host page, “College Humor,” contains porn links.

Hamilton College Streaking Team annual streak. Burke Library, Hamilton College. Article: http://www.hamilton.edu/levitt/Insights/Herring_article_2008.pdf Blog entry: http://www.jockandbuttons.com/hamiltonstreaks/gq.html (Thanks, Katherine Collett!)

Knoxville Hodges Library, University of Tennessee, May 1, 2007. http://www.youtube.com/watch?v=dX_EvqXJx44
(Thanks, Marianne Lenox!)